Human adrenergic receptors are integral membrane proteins which have been classified into two broad classes, the alpha and the beta adrenergic receptors. Both types mediate the action of the peripheral sympathetic nervous system upon binding of catecholamines, norepinephrine and epinephrine.
Norepinephrine is produced by adrenergic nerve endings, while epinephrine is produced by the adrenal medulla. The binding affinity of adrenergic receptors for these compounds forms one basis of the classification: alpha receptors tend to bind norepinephrine more strongly than epinephrine and much more strongly than the synthetic compound isoproterenol. The preferred binding affinity of these hormones is reversed for the beta receptors. In many tissues, the functional responses, such as smooth muscle contraction, induced by alpha receptor activation are opposed to responses induced by beta receptor binding.
Subsequently, the functional distinction between alpha and beta receptors was further highlighted and refined by the pharmacological characterization of these receptors from various animal and tissue sources. As a result, alpha and beta adrenergic receptors were further subdivided into xcex11, xcex12, xcex21, and xcex22 subtypes.
Functional differences between xcex11 and xcex12 receptors have been recognized, and compounds which exhibit selective binding between these two subtypes have been developed. Thus, in WO 92/0073, the selective ability of the R(+) enantiomer of terazosin to selectively bind to adrenergic receptors of the xcex11 subtype was reported. The xcex11/xcex12 selectivity of this compound was disclosed as being significant because agonist stimulation of the xcex12 receptors was said to inhibit secretion of epinephrine and norepinephrine, while antagonism of the xcex12 receptor was said to increase secretion of these hormones. Thus, the use of non-selective alpha-adrenergic blockers, such as phenoxybenzamine and phentolamine, was said to be limited by their xcex12 adrenergic receptor mediated induction of increased plasma catecholamine concentration and the attendant physiological sequelae (increased heart rate and smooth muscle contraction).
For a general background on the xcex1-adrenergic receptors, the reader""s attention is directed to Robert R. Ruffolo, Jr., xcex1-Adrenoreceptors: Molecular Biology, Biochemistry and Pharmacology, (Progress in Basic and Clinical Pharmacology series, Karger, 1991), wherein the basis of xcex11/xcex12 subclassification, the molecular biology, signal transduction, agonist structure-activity relationships, receptor functions, and therapeutic applications for compounds exhibiting xcex1-adrenergic receptor affinity was explored.
The cloning, sequencing and expression of alpha receptor subtypes from animal tissues has led to the subclassification of the xcex11 adrenoreceptors into xcex11A, xcex11B, and xcex11D. Similarly, the xcex12 adrenoreceptors have also been classified xcex12A, xcex12B, and xcex12C receptors. Each xcex12 receptor subtype appears to exhibit its own pharmacological and tissue specificities. Compounds having a degree of specificity for one or more of these subtypes may be more specific therapeutic agents for a given indication than an xcex12 receptor panagonist (such as the drug clonidine) or a panantagonist.
Among other indications, such as the treatment of glaucoma, hypertension, sexual dysfunction, and depression, certain compounds having alpha 2 adrenergic receptor agonist activity are known analgesics. However, many compounds having such activity do not provide the activity and specificity desirable when treating disorders modulated by alpha-2 adrenoreceptors. For example, many compounds found to be effective agents in the treatment of pain are frequently found to have undesirable side effects, such as causing hypotension and sedation at systemically effective doses. There is a need for new drugs that provide relief from pain without causing these undesirable side effects. Additionally, there is a need for agents which display activity against pain, particularly chronic pain, such as chronic neuropathic and visceral pain.
British Patent 1 499 485, published Feb. 1, 1978 describes certain thiocarbamide derivatives; some of these are said to be useful in the treatment of conditions such as hypertension, depression or pain.
It is an object of the invention to provide compounds and compositions useful in treating disorders modulated by alpha-2 adrenoreceptors.
It is an object of this invention to provide novel compounds having substantial analgesic activity in the treatment of chronic pain, regardless of origin. Chronic pain may be, without limitation, visceral, inflammatory, referred or neuropathic in origin. Such chronic pain may arise as a result of, or be attendant to, conditions including without limitation: arthritis, (including rheumatoid arthritis), spondylitis, gouty arthritis, osteoarthritis, juvenile arthritis, and autoimmune diseases including, without limitation, lupus erythematosus.
These compositions can also be used within the context of the treatment of chronic gastrointestinal inflammations, Crohn""s disease, gastritis, irritable bowel disease (IBD) and ulcerative colitis; and in treatment of visceral pain, including pain caused by cancer or attendant to the treatment of cancer as, for example, by chemotherapy or radiation therapy.
These compositions can be used within the context of the treatment of other chronic pain symptoms, and especially in the treatment of chronic forms of neuropathic pain, in particular, without limitation, neuralgia, herpes, deafferentation pain, and diabetic neuropathies. In a preferred embodiment these compositions are specifically analgesic in chronic pain models and do not have significant activity in acute pain models.
It is also an object of this invention to provide novel compounds for treating ocular disorders, such as ocular hypertension, glaucoma, hyperemia, conjunctivitis and uveitis.
It is also an object of this invention to provide novel compounds for treating the pain associated with substance abuse and/or withdrawal.
It is a still further object of this invention to provide such compounds which have good activity when delivered by peroral, parenteral, intranasal, ophthalmic, and/or topical dosing, or injection.
It is also an object of this invention to provide methods of treating pain through the therapeutic administration of the compounds disclosed herein.
It is further an object of the present invention to provide methods of treating conditions known to be susceptible to treatment through alpha 2 adrenergic receptors.
The present invention is directed to compounds having the formula: 
wherein R1 and R5 are independently selected from the group consisting of Cl, F, I, Br, C1-3 alkyl, C1-3 alkoxy, trifluoromethyl, hydroxyl or H, R2 and R4 are independently selected from the group consisting of Cl, F, I, Br, C1-3 alkyl, C1-3 alkoxy, hydroxyl, trifluoromethyl or H, and R3 is selected from the group consisting of F or H; and alkyl esters thereof, and pharmaceutically acceptable salts of these compounds.
The invention is also directed to methods of treating pain, particularly chronic pain, through the administration of pharmaceutically effective amounts of compounds of the above structure.
In an alternative embodiment, the invention is directed to compounds of the structure of Formula 1, excepting the compound(s) designated below as Formula 2 and/or Formula 3.
Further, the invention is directed to methods of treating glaucoma and other ophthalmic conditions (including ocular pain) through the administration of a pharmaceutically effective amount of these compounds.
In one aspect, the present invention is directed to compounds having Formula 1: 
wherein R1 and R5 are independently selected from the group consisting of Cl, F, I, Br, Cl3 alkyl, C1-3 alkoxy, trifluoromethyl, hydroxyl or H, R2 and R4 are independently selected from the group consisting of Cl, F, I, Br, C1-3 alkyl, C1-3 alkoxy, hydroxyl, trifluoromethyl or H, and R3 is selected from the group consisting of F or H; and alkyl esters thereof, and pharmaceutically acceptable salts of these compounds.
In one embodiment preferred compounds corresponding to this structure are the following compound (hereinafter termed Formula 2): 
and the following compound (hereinafter termed Formula 3): 
and their alkyl esters, and pharmaceutically acceptable derivatives and/or salts of these compounds. In an alternative embodiment, the invention is drawn to the subset of Formula 1 that excludes the compounds designated Formula 2 and/or Formula 3.
Applicants have discovered that these compounds activate xcex12 receptors, particularly xcex12B receptors. Additionally, these compounds act as a highly effective analgesic, particularly in chronic pain models, with minimal undesirable side effects, such as sedation and cardiovascular depression, commonly seen with agonists of the xcex12 receptors.
Such compounds may be administered at pharmaceutically effective dosages. Such dosages are normally the minimum dose necessary to achieve the desired therapeutic effect; in the treatment of chromic pain, this amount would be roughly that necessary to reduce the discomfort caused by the pain to tolerable levels. Generally, such doses will be in the range 1-1000 mg/day; more preferably in the range 10 to 500 mg/day. However, the actual amount of the compound to be administered in any given case will be determined by a physician taking into account the relevant circumstances, such as the severity of the pain, the age and weight of the patient, the patient""s general physical condition, the cause of the pain, and the route of administration.
The compounds are useful in the treatment of pain in a mammal; particularly a human being. Preferably, the patient will be given the compound orally in any acceptable form, such as a tablet, liquid, capsule, powder and the like. However, other routes may be desirable or necessary, particularly if the patient suffers from nausea. Such other routes may include, without exception, transdermal, parenteral, subcutaneous, intranasal, intrathecal, intramuscular, intravenous, and intrarectal modes of delivery. Additionally, the formulations may be designed to delay release of the active compound over a given period of time, or to carefully control the amount of drug released at a given time during the course of therapy.
Another aspect of the invention is drawn to therapeutic compositions comprising the compounds of Formula 1 and alkyl esters and pharmaceutically acceptable derivatives and/or salts of these compounds and a pharmaceutically acceptable excipient, or the subset of these compounds excluding Formula 2 and/or Formula 3. Such an excipient may be a carrier or a diluent; this is usually mixed with the active compound, or permitted to dilute or enclose the active compound. If a diluent, the carrier may be solid, semi-solid, or liquid material that acts as a excipient or vehicle for the active compound. The formulations may also include wetting agents, emulsifying agents, preserving agents, sweetening agents, and/or flavoring agents. If used as in an ophthalmic or infusion format, the formulation will usually contain one or more salt to influence the osmotic pressure of the formulation.
In another aspect, the invention is directed to methods for the treatment of pain, particularly chronic pain, through the administration of a compound of Formula 1 having activity against chronic pain, and pharmaceutically acceptable alkyl esters, salts, and derivatives thereof to a mammal in need thereof. In a preferred embodiment the compounds lack significant activity towards acute pain. In a further embodiment, such methods employ a compound selected from this group, but such compound is not Formula 2 or Formula 3. As indicated above, the compound will usually be formulated in a form consistent with the desired mode of delivery.
It is known that chronic pain (such as pain from cancer, arthritis, and many neuropathic injuries) and acute pain (such as that pain produced by an immediate mechanical stimulus, such as tissue section, pinch, prick, or crush) are distinct neurological phenomena mediated to a large degree either by different nerve fibers and neuroreceptors or by a rearrangement or alteration of the function of these nerves upon chronic stimulation. Sensation of acute pain is transmitted quite quickly, primarily by afferent nerve fibers termed C fibers, which normally have a high threshold for mechanical, thermal, and chemical stimulation. While the mechanisms of chronic pain are not completely understood, acute tissue injury can give rise within minutes or hours after the initial stimulation to secondary symptoms, including a regional reduction in the magnitude of the stimulus necessary to elicit a pain response. This phenomenon, which typically occurs in a region emanating from (but larger than) the site of the original stimulus, is termed hyperalgesia. The secondary response can give rise to profoundly enhanced sensitivity to mechanical or thermal stimulus.
The A afferent fibers (Axcex2 and Axcex4 fibers) can be stimulated at a lower threshold than C fibers, and appear to be involved in the sensation of chronic pain. For example, under normal conditions, low threshold stimulation of these fibers (such as a light brush or tickling) is not painful. However, under certain conditions such as those following nerve injury or in the herpesvirus-mediated condition known as shingles the application of even such a light touch or the brush of clothing can be very painful. This condition is termed allodynia and appears to be mediated at least in part by Axcex2 afferent nerves. C fibers may also be involved in the sensation of chronic pain, but if so it appears clear that persistent firing of the neurons over time brings about some sort of change which now results in the sensation of chronic pain.
By xe2x80x9cacute painxe2x80x9d is meant immediate, usually high threshold, pain brought about by injury such as a cut, crush, burn, or by chemical stimulation such as that experienced upon exposure to capsaicin, the active ingredient in chili peppers.
By xe2x80x9cchronic painxe2x80x9d is meant pain other than acute pain, such as, without limitation, neuropathic pain, visceral pain (including that brought about by Cron""s disease and irritable bowel syndrome (IBS)), and referred pain.
It has been discovered that compounds of Formula 1 have activity against chronic pain. Preferably, though not necessarily, this activity is relatively specific to chronic pain and the compounds have little activity towards acute pain. In one embodiment compounds of FIG. 1 which are monosubstituted will have either a fluorine or hydrogen substitution at the para position.
Table 1 below lists exemplary thiourea compounds and indicates, with some exceptions, their ability to modulate alpha 2A, alpha 2B and alpha 2C adrenergic receptor activity, indicated by the EC50 value (the concentration of the compound, expressed as 10xe2x88x929 moles per liter, effective to cause a modulation of receptor activity). The compounds of this invention are preferably at least 10-fold less active at the alpha 2A receptor that at the alpha 2B and/or alpha 2C receptors. While not wishing to be limited by theory, the present inventors believe that stimulation of the alpha 2A receptor is associated in mammals, including human beings, with cardiovascular effects, sedation, and diuretic activity, which activities are preferably not desired to be stimulated in compounds to be used as drugs for the treatment of chronic pain.
Activity was measured using the RSAT (Receptor Selection and Amplification Technology) assay, developed by Receptor Technologies, Inc. of Winooski, Vt. (now Acadia Pharmaceuticals of San Diego, Calif.), adapted for use with recombinant alpha2 adrenergic receptors in collaboration with Allergan. The assay measures receptor-mediated loss of contact inhibition that results in selective proliferation of receptor-containing cells in a mixed population of confluent cells. The increase in cell number is assessed with an appropriate transfected marker gene such as xcex2-galactosidase, the activity of which can be easily measured in a 96-well format. Receptors that activate the G protein, Gq, elicit this response. Alpha2 receptors, which normally couple to Gi, activate the RSAT response when coexpressed with a hybrid Gq protein that has a Gi receptor recognition domain, called Gq/i5.
The assay is conducted as follows. NIH-3T3 cells are plated at a density of 2xc3x97106 cells in 15 cm dishes and maintained in Dulbecco""s modified Eagle""s medium supplemented with 10% calf serum. One day later, cells are cotransfected by calcium phosphate precipitation with mammalian expression plasmids encoding p-SV-xcex2-galactosidase (5-10 xcexcg), receptor (1-2 xcexcg) and G protein (1-2 xcexcg). 40 xcexcg salmon sperm DNA may also be included in the transfection mixture to increase transfection efficiency. Fresh media is added on the following day and 1-2 days later, cells are harvested and frozen in 50 assay aliquots. Transfectant cells are thawed and 100 xcexcl added to 100 xcexcl aliquots of various concentrations of drugs in triplicate in 96-well dishes. Incubations continue 72-96 hr at 37xc2x0. After washing with phosphate-buffered saline, xcex2-galactosidase enzyme activity is determined by adding 200 xcexcl of the chromogenic substrate (consisting of 3.5 mM o-nitrophenyl-xcex2-D-galactopyranoside and 0.5% of the non-ionic surfactant nonidet P-40 in phosphate buffered saline), incubating overnight at 30xc2x0 and measuring optical density at 420 nm. The absorbency is a measure of enzyme activity, which depends on cell number and reflects a receptor-mediated cell proliferation. The EC50 and maximal effect of each drug at each alpha2 receptor is determined. The efficacy or intrinsic activity is calculated as a ratio of the maximal effect of the drug to the maximal effect of a standard full agonist for each receptor subtype. Brimonidine, also called UK14304, is used as the standard agonist for the alpha2A, alpha2B and alpha2C receptors. The RSAT assay is also discussed in Messier et al. (1995) High throughput assays of cloned adrenergic, muscarinic, neurokinin and neurotrophin receptors in living mammalian cells, Pharmacol. Toxicol. 76:308-11; see also Conklin et al. (1993) Substitution of three amino acids switches receptor specificity of Gqxcex1 to that of Gixcex1, Nature 363:274-6; Both of these papers are incorporated by reference herein.